I use Colourmailer as well. They do fiddle with the colour profile though unless you select the option to not do it. It's only in their new software. The old app doesn't give you the ability to change that option at all.

I've also got an account with the Colourmailer Pro service as I print out quite a few big posters and it's cheaper as they do discounts.

I've used Bonusprint on occasion as well but the quality wasn't as good as Colourmailer but it was a bit cheaper and slightly faster to turn up.

The instore mini labs that you get in Tesco, ASDA etc... aren't very good either in my opinion. I've tried printing stuff with them and it comes out a little fuzzy even when the original isn't. The kiosk things have terrible quality and are only suitable if you don't know what a proper print is capable of or you're in a hurry.